The present invention relates, in general, to semiconductor circuits and, more particularly, to multiplier circuits.
Multiplier circuits are well known in communication applications. A multiplier combines a radio frequency (RE) signal and a local oscillator (LO) signal to generate a mixed signal, which has a spectrum comprised of the sum and difference of the frequencies of the RF and LO signals. If the high frequency components of the mixed signal are filtered out, the output signal of the multiplier is usually an intermediate frequency (IF) signal.
A Gilbert multiplier is widely used as a multiplier circuit because of its good performance parameters, such as small intermodulation product distortions, low noise figures, etc. A large current bias to a Gilbert multiplier with an emitter degeneration resistor improves the linearity of the Gilbert multiplier. The voltage swing of the output signal of the Gilbert multiplier is proportional to the current bias. A large current bias also results in a large voltage swing. In addition, a conventional Gilbert multiplier includes a cross-coupled pair and an emitter-coupled pair serially coupled between two supply voltage levels. The emitter degeneration resistor, the large voltage swing, the serial coupling of the cross-coupled pair and the emitter-coupled pair require a high supply voltage for the Gilbert multiplier to operate properly. Therefore, a conventional Gilbert multiplier is not suitable for use in low voltage applications.
Accordingly, it would be advantageous to have a multiplier and a method for mixing signals. It is desirable for the multiplier and the method to be suitable for use in low voltage applications. It is also desirable for the multiplier to have good signal isolation.